@Prince of Seraphs Problem with advance is it's so... Out there. Even I won't go into it because I don't think I can write enough reliably. Thus I'd rather stay in casual.
That's a reason I don't go there. I think I'm fully capable of performing as Advanced requests but I don't want to be
required to do that well all of the time. It takes a fair bit of time and energy. In casual if I have the time and energy to spend I can crank out an Advanced level post. If not no one is going to yell at me for doing a little less.
As far as making people stay... Really I think that it's about attracting people that are sincerely interested from those people that see the Original Post think "this might be fun" then decide it's too much work. The best way to do that I would say is to make it more difficult to get accepted. Put something in the Original Post about 'acceptance not guaranteed' or 'acceptance dependent on quality of Character Sheet' Perhaps make them include a sample post to test there style of writing. If people believe they are being heavily scrutinized when applying those that are joining just for the hell of it and will probably leave in a couple weeks won't bother because it's too much of a hassle while those that really wish to be here will perform better in order to get accepted.
Another way to go would be to make the Character Sheets more inclusive. Add categories unique to this RP that require some serious thought. Anyone will dash out a basic character sheet based on categories more or less universal to to Roleplaying but if they look at the form see something they are not used to dealing with and there first thought is "I'm gonna have to think about this one" those that don't really care will move on while those that really want to be here will put some effort into it.
Lastly I think we lost a number of people because they were uncertain of their actions. This is supposed to be a more freeroam style RP where the plot is driven by the actions of the characters. It's comparable to an open world video game where you don't have one set story path you must follow. The difficulty with that kind of game is that a programmer must anticipate any action a player might choose to take and plan how the game will respond. They have to design settings, NPCs, general rules, that kind of thing. I think you left things just a little too freeform. The RPers didn't have enough established (for lack of a better term) 'Overworld' to work with. Have you ever read the novel Coraline or watched the movie adaptation? In the other world outside the house and grounds the world is entirely white and blank. Coraline walks in a straight line away from the house and comes back on it again and comments that it must be a small world indeed for her to be able to walk all the way around it in such a short time.
At the moment the Fae Courts look a bit like that. Blank and formless. We know for sure that the courts are more or less populated by forests, each one a little different depending on the season it embodies and we have a rough description of each of the palaces from the various rulers before they dropped out but we don't have anything more concrete than that. I've tried to take what liberties I could but even I'm a little in the dark and no one wants to make anything too drastic like an entire city for fear that it might be out of line with what you (the GM) have in mind. Are there cities in the four courts? What are they like? Is there more than one or does each court just a have a capital? What does infrastructure look like? Are there roads running through the forest? Are they stone paved roads or just paths worn into the ground from millennium of travel? Are the cities made with stone buildings like a medieval city or sung out of the trees like some fae depictions?
I took some more extreme liberties with how the courts borders are guarded by establishing a barrier between the four courts that is only crossable at certain predetermined gates except by using the blood of the Tuatha but prior to that it could have been simply that one season bled into the next and no one patrolled the area.
I've done my best to use popular Welsh mythology to fill in the gaps but there is very little literature that deals specifically with what Faerie is like. Most of it deals with specific fairies and there actions in the mortal realm. If we are going to Roleplay in the world of the Fae we need some idea as to what that world is actually like. Maybe if you put some basic information about what each of the courts is like and perhaps some common destinations for the rulers to work with that would help immensely.
I think I went off on a bit of a tangent there at the end but that's my two cents. Take of it what you will.